Several kinds of species are known in Chlamydia, that is, Chlamydia trachomatis, Chlamydia psittaci, Chlamydia pecorum, Chlamydia pneumoniae and the like. Chlamydia trachomatis causes trachoma, venereal lymphogranuloma, urogenital infections, inclusion conjunctivitis, neonatal pneumonia and the like. Chlamydia psittaci causes psittocosis and the like. Chlamydia pneumoniae causes respiratory infections, atypical pneumonia and the like.
Since the symptoms of infections in the respiratory apparatus which are caused by Chlamydia pneumoniae are similar to those of infections caused by Mycoplasma pneumoniae or Influenza virus, physicians often make a wrong diagnosis. Hence, there is a need for the development of a simple method for diagnosing the infections caused by Chlamydia pneumoniae.
In general, an infection can reliably be diagnosed by detecting the causative bacterium in the infected site or by detecting an antibody against the causative bacterium in body fluids such as a sera and the like. The former method is called an antigen test and the latter is called an antibody test. Both of them are clinically important. As for Chlamydia pneumoniae, there is known an antibody test which is carried out by a method in which an antibody is detected by using an elementary body of Chlamydia pneumoniae.
However, this method has the disadvantage that the elementary body of Chlamydia pneumoniae reacts not only with an antibody against Chlamydia pneumoniae but also with antibodies against other species of Chlamydia, thus being fairly unspecific. This is because the elementary body of Chlamydia pneumoniae contains an antigen which is also present in other species of geneus Chlamydia than Chlamydia pneumoniae, that is, Chlamydia trachomatis and Chlamydia psittaci.
As a plasmid which can be used for the expression of a large amount of a protein in E. coli, pBBK10MM is known (Japanese Unexamined Patent Publication No. Hei 4-117284). This plasmid can be used for the expression of a fused protein of an anti-allergic peptide with DHFR. The expressed fused protein also maintains the enzymatic activity of DHFR and can therefore be purified easily by utilizing the characteristic properties and activities of DHFR.
Genetic screening has been carried out to diagnose infections. In this screening, the presence of the gene of a microorganism to be detected in a sample is examined using nucleic acid probes and the like.
As for Chlamydia pneumoniae, there is known a genetic screening method which is carried out as disclosed in Japanese Unexamined Patent Publication No. Sho 64-500083, U.S. Pat. No. 5,281,518 and WO94/04549.
However, Japanese Unexamined Patent Publication No. Sho 64-500083 and U.S. Pat. No. 5,281,518 only disclose that a chromosomal DNA of Chlamydia pneumoniae or a DNA fragment which is obtained by cleaving the chromosomal DNA with a restriction enzyme or the like is used as a probe. The base sequences of these DNA molecules are not determined and the specificity of these probes are therefore unclear. In addition, it is difficult to determine the reaction conditions.
Although WO94/04549 discloses a method using a probe which is hybridized to ribosome RNA or DNA corresponding thereto, the specificity of these probes is not reliable because the homology of ribosomal RNA is relatively high in all organisms.